Temperature compensation mechanism for a micromechanical ring resonator

ABSTRACT

A time base including a resonator ( 4 ) and an integrated electronic circuit ( 3 ) for driving the resonator into oscillation and for producing, in response to the oscillation, a signal having a determined frequency. The resonator is an integrated micromechanical ring resonator supported above a substrate ( 2 ) and adapted to oscillate around an axis of rotation (O) substantially perpendicular to the substrate. The ring resonator includes a free-standing oscillating structure having a plurality of thermally compensating members ( 65 ) which are adapted to alter a mass moment of inertia of the free-standing oscillating structure as a function of temperature so as to compensate for the effect of temperature on the resonant frequency of the ring resonator.

RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.10/129,193 (now U.S. Pat. No. 6,686,807) filed on May 2, 2002 in thename of Metin GIOUSOUF et al. and entitled “Time base comprising anintegrated micromechanical ring resonator” which is assigned to thepresent Assignee, and which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a time base, i.e. a device comprising aresonator and an integrated electronic circuit for driving the resonatorinto oscillation and for producing, in response to this oscillation, asignal having a determined frequency as well as to a resonator for usein such a time base. The present invention more particularly relates toa compensation mechanism for compensating for the effect of temperatureon the resonant frequency of the resonator.

BACKGROUND ART

Time bases, or frequency standards, are required in a large variety ofelectronic devices, ranging from wristwatches and other timepieces tocomplex telecommunication devices. Such time bases are typically formedby an oscillator including a quartz resonator and an electronic circuitfor driving the resonator into oscillation. An additional division chainmay be used to divide the frequency of the signal produced by theoscillator in order to obtain a lower frequency. Other parts of thecircuit may serve to adjust the frequency, for example by adjusting thedivision ratio of the division chain. The components of the electroniccircuit are advantageously integrated onto a single semiconductorsubstrate in CMOS technology. Other functions, not directly related tothe frequency processing, may be integrated onto the same substrate.

Advantages of quartz resonators are their high quality factor Q leadingto good frequency stability and low power consumption as well as theirgood temperature stability. A disadvantage of typical time bases usingquartz resonators however resides in the fact that two components,namely the quartz resonator and the integrated electronic circuit, arerequired in order to provide a high-precision frequency. A discretequartz resonator requires board space which is scarce in many cases. Forinstance, a standard quartz resonator for wristwatch applicationsrequires space of the order of 2×2×6 mm³. Moreover, additional costs arecaused by the assembly and connection of the two components. Yet, spaceand assembly costs are major issues, especially in the growing field ofportable electronic devices.

A solution to the above-mentioned problems is to provide a time basecomprising an integrated resonator.

More particularly, one solution consists in providing a time basecomprising a resonator and an integrated circuit for driving theresonator into oscillation and for producing, in response to theoscillation, a signal having a determined frequency, the resonator beingan integrated micromechanical ring resonator supported above a substrateand adapted to oscillate around an axis of rotation substantiallyperpendicular to the substrate, this ring resonator comprising:

-   -   a central post extending from the substrate along the axis of        rotation    -   a free-standing oscillating structure connected to the central        post and including an outer ring coaxial with the axis of        rotation and connected to the central post by means of a        plurality of spring elements; and    -   electrode structures disposed around the outer ring and        connected to the integrated electronic circuit. Advantages on        this solution reside in the fact that the time base may be fully        integrated on a single substrate, is suitable for mass        production and is compatible with CMOS technology. In addition,        such a time base is low-priced and requires only a very small        surface area on a semiconductor chip.

An advantage of this time base lies in the fact that the micromechanicalring resonator exhibits a high quality factor Q. Quality factors as highas 2×10⁵ have been measured. For comparison, tuning-fork quartzresonators usually exhibit values between 5×10⁴ and 1×10⁵ after lasertrimming of the fork tines. Different design features favouring a highquality factor Q are proposed.

In addition, for a given resonant frequency, the surface area requiredon the substrate to form the ring resonator is small in comparison withother resonators.

The electronic circuit may advantageously be integrated on the substratetogether with the micromechanical ring resonator, thereby leading to alow-priced time base. A lower price is also obtained by wafer-levelpackaging of the resonator using wafer-bonding technology.

It must be pointed out that ring resonators having similar features areknown from sensing devices, such as angular rate sensors, accelerometersor gyroscopes. For instance U.S. Pat. No. 5,450,751 to Putty et al. andU.S. Pat. No. 5,547,093 to Sparks both disclose a micromechanical ringresonator for a vibratory gyroscope comprising a plated metal ring andspring system supported above a silicon substrate. U.S. Pat. No.5,872,313 to Zarabadi et al. discloses a variant of the above sensorwhich is configured to exhibit minimum sensitivity to temperaturevariation. U.S. Pat. No. 5,025,346 also discloses a ring resonator foruse as a micro-sensor in a gyroscope or an angular rate sensor.

None of the above-cited documents however indicates or suggests usingsuch a type of ring resonator in an oscillator circuit to act as afrequency standard or time base. Moreover, a number of design features(e.g. the shape and number of spring elements) of the ring resonatorsdisclosed in these documents are such that they would not be suitablefor horological applications where frequency stability and low powerconsumption are essential. For instance, the resonating structuresdisclosed in U.S. Pat. No. 5,025,346 exhibit a quality factor rangingfrom 20 to 140 which is too low for being used in a highly precise timebase in horological applications, whereas quartz resonators used inhorological applications exhibit quality factors of the order of 1×10⁴to 1×10⁵.

Within the scope of the above solution, various design features areproposed which lead to a high quality factor Q, a high stability of theoscillation frequency against variations in the amplitude of the drivingvoltage, and tolerance of fabrication process variations. In fact, oneof the major objectives for an application as an oscillator is a highquality factor Q. A high quality factor Q results in a stableoscillation with low phase noise and low power consumption, as isrequired for horological applications.

One problem of the above solution however resides in the effect oftemperature on the resonant frequency of the resonator. The resonantfrequency of the ring resonator is, within the temperature range of 0 to60° C., in good approximation, a linear function of temperature. At aresonant frequency of 45 kHz, it has been observed that the thermalcoefficient of the resonant frequency is of the order of −25 ppm/° C.

Two main factors determine the temperature characteristics of the ringresonator. Firstly, Young's modulus E of the material used to realizethe vibrating structure decreases with increasing temperature resultingin a reduced stiffness of the spring elements and therefore a lowerresonant frequency. Secondly, due to thermal expansion, the diameter ofthe ring will increase with increasing temperature resulting in anincreased mass moment of inertia of the structure, which, in turn, alsoreduces the resonant frequency.

One solution to the above problem may consist in integrating atemperature measuring circuit on the substrate in order to compensatefor the effect of temperature on the frequency of the signal produced bythe time base. Such compensation of the resonator's temperaturedependency may easily be effected since the above ring resonator has theadvantage of exhibiting substantially linear temperaturecharacteristics.

Another solution to the above problem may consist in forming a secondmicromechanical ring resonator on the substrate in order to allowtemperature compensation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a mechanism forsubstantially compensating for the effect of temperature on the resonantfrequency of the ring resonator which does not require an additionaltemperature measuring circuit or an additional resonator.

Accordingly, there is provided a time base, as well as a resonator, ofthe above-mentioned type wherein the free-standing oscillating structureof the resonator further comprises a plurality of thermally compensatingmembers, these thermally compensating members being adapted to alter amass moment of inertia of the free-standing oscillating structure as afunction of temperature so as to compensate for the effect oftemperature on the resonant frequency of the ring resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features and advantages of the present invention will beapparent upon reading the following detailed description of non-limitingexamples and embodiments made with reference to the accompanyingdrawings, in which:

FIG. 1 is a top view illustrating schematically a first embodiment of atime base comprising a micromechanical ring resonator and an integratedelectronic circuit;

FIG. 2 is a detailed view of the central post of the micromechanicalring resonator and its junctions with the spring elements;

FIG. 3 is a detailed view of a portion of the outer ring with itsjunctions with the spring elements;

FIG. 4 is a cross-sectional view of the micromechanical ring resonatorof FIG. 1 taken along line A-A′;

FIG. 5 shows an idealized straight spring element with a section of theouter ring;

FIG. 6 shows a top view illustrating schematically a second embodimentof a time base;

FIGS. 7 a to 7 c show detailed top views of three different designsintended to prevent the ring resonator from sticking on the electrodestructures;

FIG. 8 shows a top view illustrating an improvement of the firstembodiment shown in FIG. 1;

FIG. 9 is a cross-sectional view of the embodiment of FIG. 8 taken alongline A-A′;

FIGS. 10 a and 10 b are two top views illustrating two variants of amechanism according to the present invention for altering the massmoment of inertia of the ring resonator as a function of temperature, inorder to substantially compensate for the effect of temperature on theresonant frequency of the ring resonator;

FIGS. 11 a and 11 b are respectively top and cross-sectional viewsillustrating a second mode of oscillation where the resonator performs atilting oscillation; and

FIGS. 12 a and 12 b are respectively top and cross-sectional viewsillustrating another second mode of oscillation where the resonatorperforms a vertical oscillation perpendicular to the substrate plane.

EMBODIMENTS OF THE INVENTION

FIG. 1 schematically shows a top view of a first embodiment of a timebase. There is shown an integrated time base, indicated generally byreference numeral 1, comprising a resonator 4 and an integratedelectronic circuit 3 for driving the resonator into oscillation and forproducing, in response to this oscillation, a signal having a determinedfrequency. FIG. 4 shows a cross-sectional view of the ring resonator 4taken along line A-A′ as shown in FIG. 1.

The integrated electronic circuit 3 is not shown in detail since thiscircuit may easily be designed by those skilled in the art. Preferablyboth the integrated electronic circuit 3 and the resonator 4 arerealized and integrated on a same substrate 2 as illustrated in FIG. 1.A preferred substrate material is silicon, but other similar materialsknown by those skilled in the art to be equally suitable for realisingthe time base of the present invention may be used.

The resonator 4 is realised in the form of a monolithic micromechanicalresonating ring, hereinafter referred to as a micromechanical ringresonator, which is essentially supported above the substrate 2 andadapted to oscillate around an axis of rotation O substantiallyperpendicular to the substrate 2. The ring resonator 4 essentiallycomprises a central post 5 extending from the substrate 2 along the axisof rotation O and a free-standing oscillating structure, indicatedglobally by reference numeral 6, connected to the central post 5.

The free-standing oscillating structure 6 includes an outer ring 60coaxial with the axis of rotation O, and a plurality of spring elements62 disposed symmetrically around the central post 5 and connecting theouter ring 60 to the central post 5. The spring elements 62 areessentially formed as curved rod-shaped spring elements. It will beappreciated that the central post 5 constitutes the only mechanicalconnection of the ring resonator 4 with the substrate 2 and thatoscillation of the resonator takes place in a plane substantiallyparallel to the surface of the substrate 2.

The ring resonator 4 further comprises pairs of diametrically opposedelectrode structures surrounding the outer ring 60, indicated byreference numeral 9 in FIG. 1. According to this first embodiment,comb-shaped members 8 are provided on the outer ring 60 of thefree-standing oscillating structure 6. These comb-shaped members 8 forma part of the electrode structures of the ring and each include a basemember 80 extending radially from the outer ring 60 and first and secondlateral members, indicated respectively by reference numerals 82 and 84,that extend substantially perpendicularly from both sides of the basemember 80.

The electrode structures 9 comprise first and second comb-shapedelectrode structures 91 and 93 surrounding the outer ring 60 in such away that they mesh with the comb-shaped members 8 of the free-standingoscillating structures. More particularly, according to this embodiment,the first comb-shaped electrode structure 91 includes first electrodes92 and meshes with comb-shaped member 8 so that the first electrodes 92are adjacent to the first lateral members 82. Similarly, the secondcomb-shaped electrode structure 93 (disposed opposite the firstcomb-shaped electrode structure 91) includes second electrodes 94 andmeshes with comb-shaped member 8 so that the second electrodes 94 areadjacent to the second lateral members 84. As shown in FIG. 1, thelateral members 82, 84 and the electrodes 92, 94 of the first and secondelectrode structures 91, 93 are preferably designed so as to have theshape of an arc of a circle concentric with the outer ring 60.

In this embodiment, the first comb-shaped electrode structures 91 serveto electrostatically drive the ring resonator 4 into oscillation, andthe second comb-shaped electrode structure 93, which are disposed on theother side of the base members 80, serve to capacitively sense theoscillation of the resonator. The first electrode structures 91surrounding the resonator 4 are connected together via a first conductor11 formed on the substrate 2, and, similarly, the second electrodestructures 93 are connected together via a second conductor 12 formed onthe substrate 2. These conductors 11, 12 as well as a third conductor 13providing an electrical contact to the ring via the central post 5 areconnected to appropriate terminals of the electronic circuit 3.

FIG. 4 shows a cross-sectional view of the ring resonator 4 taken alongline A-A′ as illustrated in FIG. 1. Thickness and other dimensions arenot to scale. There is shown the substrate 2, the central post 5 alongthe axis of rotation O of the ring resonator, the free-standingoscillating structure 6 including the outer ring 60 and the springelements 62, the lateral members 82 of the comb-shaped members 8, theelectrodes 92 of the first comb-shaped electrode structures 91, and thefirst and second connectors 11, 12 that respectively connect theelectrode structures 91 and 93 surrounding the outer ring 60. FIG. 4further shows a first insulating layer 20, such as a silicon oxidelayer, formed above the surface of substrate 2, beneath the ringresonator 4 and onto which are formed the first and second conductors11, 12. A second insulating layer 21, such as another oxide layer orsilicon nitride layer, is formed above the first layer 20 below the ringresonator.

The resonating ring structure is preferably manufactured by means ofsilicon surface micro-machining techniques which are familiar to thoseskilled in the art and will therefore not be described here. One suchtechnique makes use of a poly-silicon layer deposited on top of aso-called “sacrificial layer” in order to form the free-standingstructures of the resonator. Another technique uses a buried oxidelayer, such as e.g. in a silicon on insulator (SOI) wafer, as thesacrificial layer and results in a free-standing structure made ofmono-crystalline silicon. Other material and processing techniques,however, may also be used to realise the micromechanical ring resonatoraccording to the present invention.

One of the major objectives for an application as a time base orfrequency standard is a high quality factor Q of the resonator. A highquality factor Q results in a stable oscillation with low phase noiseand low power consumption as is required for horological applications.The quality factor Q of the micromechanical ring resonator is very highdue to a number of advantageous design features that will be explainedbelow. As already mentioned hereinabove, quality factors as high as2×10⁵ have been measured on these structures. For comparison,tuning-fork quartz resonators usually exhibit values between 5×10⁴ and1×10⁵ after laser trimming of the fork tines.

The shape of the spring elements 62 connecting the outer ring 60 to thecentral post 5 is optimised so as to obtain a high quality factor Q. Incontrast to the conditions present when using straight spring elements,the tensions along the bending line are, in the present case,homogeneously distributed along the spring element. The curved shape issuch that energy losses per oscillation period are kept minimal.

In addition, junctions 63 of the spring elements 62 with the centralpost 5 are substantially perpendicular, as shown in FIG. 2. Preferably,round shapes or fillets 63 a are provided at the junctions 63. Thesefillets 63 a prevent notch tensions during oscillation, therebyfavouring an elevated quality factor Q, as substantially no energy isdissipated in the central post 5 during oscillation. Furthermore, thecentral post 5 remains substantially free of tension, which againfavours a high quality factor Q. FIG. 3 shows the junctions 64 of thespring elements 62 with outer ring 60. Here also, substantiallyperpendicular junctions 64 and fillets 64 a are preferred designs.

Using a plurality of spring elements 62 rather than the minimum of threerequired for a well-defined suspension increases the quality factor Q.Due to the fact that minor geometrical variations (e.g. as a result ofspatial fluctuations in processing) as well as material inhomogeneitiesare averaged over the plurality of spring elements, the quality factor Qincreases with the number of spring elements. The upper limit is givenby geometrical restrictions due to the design rules of themicro-structuring process. The number of spring elements is thereforecomprised between four and fifty, and preferably is of the order oftwenty.

Another element favouring a high quality factor Q of the ring resonatoris the perfect rotationally symmetrical structure, where the centre ofgravity of the entire structure remains motionless. Non-linear effects,present in most other resonator designs, are thereby removed to a largeextent.

The resonant frequency of the ring resonator can be adjusted over a widerange by changing the geometrical dimensions of the device. The ringresonator can be looked at as a plurality of spring elements connectedto a segment of the outer ring. In a zero-order approximation, and inorder to obtain a close algebraic expression for the resonant frequency,one can study the case of a straight spring element 22 with a segment 27of the outer ring 60, as shown in FIG. 5. The resonant frequency f_(r)of this structure reads:$f_{r} \approx {\frac{1}{2\pi}\sqrt{\frac{3 \cdot E \cdot J}{l^{3}\left( {m_{r} + {0.24 \cdot m_{s}}} \right)}}}$where J=d·w³/12 is the surface moment of inertia of the structure, E isthe elasticity module, d, w and l are the thickness, width and length ofthe straight spring element 22, respectively, and m_(r), m_(s) are themasses of the ring segment 27 and spring element 22, respectively. Itcan be easily seen from the above formula, that the resonance frequencycan be influenced by varying the width and/or length of the springelements or by varying the mass of the outer ring (including the mass ofthe comb-shaped members 8), again via its geometrical dimensions.Scaling of the entire structure further widens the accessible frequencyrange.

It is important for mass production of such ring resonators to keep theresonant frequency from one chip to the other within small tolerances.Tolerances in the resonant frequency due to slight variations in processparameters can be greatly reduced by carefully dimensioning the ring andsprings. This can again be shown using the example of FIG. 5. Theresonant frequency will be lower than the projected frequency if thewidth of the spring elements 22, indicated by reference numeral 26, issmaller after processing, e.g. due to an over-etch, than a desired width25. However, if one considers that at the same time the mass of the ring60 (as well as the mass of the base members 80 and lateral members 82,84) is lowered due to the same over-etch, the decrease of the resonantfrequency will be compensated for by the reduction of the masses.Openings in the ring and the bars (not shown in the Figures), which maybe necessary for processing the structure, favour this effect.

The surface area required by the micromechanical ring resonator is verysmall with respect to the resonant frequency obtained. For instance, aring resonator designed for a rather low frequency of 32 kHz requires asurface of well below 1 mm². Conventional structures require relativelylarge structures in order to obtain such a low frequency. For a givengeometrical layout, the dimensions and frequency are inversely related,i.e. the larger the geometrical dimensions, the lower the frequency. Forcomparison, EP 0 795 953 describes a silicon resonator requiring asurface of about 1.9 mm² for a higher frequency of 1 MHz. It is obviousthat the substrate surface area required by the resonator is directlyrelated to the price of the integrated time base.

The resonant frequency of the ring resonator is, within the temperaturerange of 0 to 60° C., in good approximation, a linear function oftemperature. At a resonant frequency of 45 kHz, it has been observedthat the thermal coefficient of the resonant frequency is of the orderof −25 ppm/° C. It is thus desirable to incorporate, in the samesubstrate 2, a temperature measuring circuit having an output signalwhich may be used to compensate for the frequency variation byadequately adjusting the frequency of the signal produced by the timebase.

To this effect, the time base may advantageously comprise an integratedtemperature measuring circuit (not shown). An example of such atemperature measuring circuit is described in the article “SmartTemperature Sensor in CMOS Technology” by P. Krumenacher and H. Oguey,in “Sensors and Actuators”, A21-A23 (1990), pages 636 to 638. Here,temperature compensation is achieved by acting on the division ratio ofthe division chain, for instance using an inhibition technique wellknown to those skilled in the art.

Alternatively, two ring resonators with different resonant frequenciesmay be integrated onto the same chip, such arrangement allowing the chiptemperature to be precisely determined by measuring the frequencydifference of the two resonators (both ring resonators have the sametemperature coefficient since they are made from the same material).

The advantage of using integrated time bases as described hereinabove istwofold: Firstly, the temperature dependency of the ring resonator islinear which facilitates the electronic signal treatment necessary tocompensate for the temperature. Secondly and more importantly, the smallsize and monolithic integration of the ring resonator allows a secondresonator to be provided with only a slight increase in chip size andwithout further external connections.

Alternatively, it is possible to use a single ring resonator whichoperates simultaneously with two oscillation modes. A first of thesemodes is the above described rotational mode. A second oscillation modemay be a tilting oscillation mode, wherein the free-standing structure 6performs a tilting oscillation against the substrate plane. This tiltingoscillation mode may be excited electrostatically and sensedcapacitively by using further electrodes on the substrate under the ringarea. The two modes are selected to have different frequencies so thattemperature compensation may be achieved by measuring the frequencydifference. A schematic illustration of the above mentioned tilt mode isshown in FIGS. 11 a and 11 b. As shown in these figures, two sets ofelectrodes 100 and 120 (in this case four) having substantially theshape of arcs of circles are disposed on the substrate under the ring 60so that the first set of electrodes 100 drives the structure 6 into atilting oscillation and the second set of electrodes 120 senses thistilting oscillation. The set of driving electrodes 100 and the set ofsensing electrodes 120 are disposed on opposite sides of the structure 6with respect to the central post 5 (respectively on the left and rightsides in FIG. 11 a).

A second oscillation mode may be a vertical oscillation mode, whereinthe free-standing structure 6 performs a vertical oscillationperpendicular to the substrate plane, i.e. the free-standing structure 6oscillates in a direction parallel to the axis of rotation O. Aschematic illustration of the above mentioned perpendicular mode isshown in FIGS. 12 a and 12 b. As shown in these figures, two sets ofelectrodes 130 and 150 are disposed on the substrate under the ring 60so that the first set of electrodes 130 drives the structures 6 into anoscillation perpendicular to the substrate plane and the second set ofelectrodes 150 senses this oscillation. In contrast to the tilting mode,the set of driving and sensing electrodes 130, 150 are disposedsymmetrically around the central post 5, i.e. the sets of electrodeseach comprise diametrically opposed electrodes.

As already mentioned, the comb-shaped electrode structures 91 shown inthe embodiment of FIG. 1 serve to electrostatically drive the ringresonator into oscillation and the opposite comb-shaped electrodestructures 93 serve to capacitively sense this mechanical oscillation.An alternating voltage signal is applied to electrode structures 91resulting in electrostatic forces on the ring and oscillation thereof,which, in turn, induces an alternating signal on the opposite set ofelectrode structures 93, when the resonator operates. It will beunderstood that electrode structures 91 and 93 are interchangeable.

Since there is a parabolic relationship between the voltage applied onthe electrodes and the resulting force on the ring and, it is desirableto add a constant direct voltage to the alternating voltage so as toobtain a substantially linear force-voltage relationship. In theschematic representation of FIG. 1, there are shown three signal linesor conductors 11 to 13 that are respectively connected to electrodestructures 91, electrode structures 93 and central post 5. These linesserve to drive the ring resonator into oscillation and to sense thisoscillation via the respective electrode structures.

According to a first variant, conductor 13 may be used to apply thedirect voltage component to the ring resonator via the central post 5,while the alternating voltage component is applied to electrodestructures 91 via conductor 11, conductor 12 being used to sense theresulting signal. According to a second variant, the alternating drivingvoltage and the direct voltage component may be superposed on electrodestructures 91 via conductor 11 while the ring resonator is tied to afixed potential, such as e.g. ground, via conductor 13. Conductor 12 isused to sense the signal in this case. It will be appreciated thatelectrode structures 91 and 93 are interchangeable and that electrodestructures 93 may alternatively be used for driving, electrodestructures 91 being used for sensing.

Alternatively, sensing may be done by detecting a change in impedance atresonance. As represented in FIG. 6, such a solution requires only twoconductors, 11 and 13, and an electrode structure 9* comprising a singleset of comb-shaped electrode structures 91 connected to conductor 11(the comb-shaped members 8* are modified accordingly and only comprisefirst lateral members 82). According to a first variant, the alternatingdriving voltage is applied, via conductor 11, to the single set ofelectrode structures 91, and the direct voltage component is applied tothe ring via conductor 13. According to another variant, the sum ofalternating and direct driving voltages can be applied to electrodestructures 91 via conductor 11, the ring being in this case tied viaconductor 13 to a fixed potential such as e.g. ground.

The two-conductor option provides two advantages, namely (i) a reductionin the diameter of the entire structure since a second conductor and asecond set of electrode structures surrounding the ring is no longerrequired, and (ii) the possibility of providing a larger number ofcomb-shaped electrode structures 91 along the periphery of the outerring 60, resulting in an enhanced signal.

The different modes of operation of the ring resonator are summarized inthe following table. It will be appreciated that, in any of theabove-mentioned variants, the signals applied to the driving electrodesand the ring, namely the alternating driving voltage and the directvoltage component, are perfectly interchangeable.

Electrodes 91 Ring Electrodes 93 Remarks 3 Conductors AC-driving DC-biasSensing Electrodes 91 AC-driving + Fixed potential, Sensing and 93 areDC bias e.g. ground interchangeable DC-bias AC-driving Sensing Fixedpotential, AC-driving + Sensing e.g. ground DC-bias 2 ConductorsAC-driving DC-bias — Sensing is done AC-driving + Fixed potential, — bydetecting a DC bias e.g. ground change in DC-bias AC-driving — impedanceat Fixed potential, AC-driving + — resonance e.g. ground DC-bias

The fact that the lateral members 82, 84 and the electrodes 92, 94 areof curved shape and concentric with outer ring 60 reducesnon-linearities in the electro-mechanical coupling, resulting in a highquality factor Q on the one hand and a resonant frequency of the ringresonator which is essentially independent of the amplitude ofalternating and direct driving voltages on the other hand. Furthermore,the micromechanical ring resonator can be driven with voltages as low as1.5 V, which is a major advantage for portable electronic applications.

In addition, due to electrostatic driving and capacitive sensing, anddue to the high quality factor Q determined by the design, the powerconsumption of the ring resonator is ten to hundred times lower thanthat of a quartz, which is of particular interest for portableelectronics applications.

FIGS. 7 a to 7 c show three different advantageous design featuresintended to prevent the ring resonator from sticking in case of a shock.According to a first variant shown in FIG. 7 a, stop structures 28disposed on the substrate 2 are provided at outer ends 80 a of the basemembers 80. These stop structures 28 are designed so as to limit theangular movement of the ring structure 6 and therefore prevent thefree-standing oscillating structure 6 from sticking on the electrodestructures 9 when excessive angular movements take place due, forinstance, to mechanical shocks.

Alternatively, as shown in FIG. 7 b, extremities 82 a, 84 a of thelateral members 82, 84 and/or extremities 92 a, 94 a of the electrodes92, 94 may be designed so as to exhibit a pointed shape or at least asuitably small surface area so as to prevent sticking.

Finally, as shown in the variant of FIG. 7 c, one 82*, 84* of thelateral members 82, 84 can be made longer than the others, therebyreducing the adhesion forces when the comb-shaped members 8 and thecomb-shaped electrode structures 91, 93 get into mechanical contact witheach other. Obviously, the same effect may be obtained when one ofelectrodes 92 and 94 is longer than the others.

FIGS. 8 and 9 show an improvement of the micromechanical ring resonator4 which is illustrated in FIG. 1. FIG. 9 shows a cross-sectional view ofFIG. 8 taken along line A-A′. A conductive pattern 31 is provided on (orbelow) the surface of the substrate 2 under at least part of thefree-standing oscillating structure 6, i.e. spring elements 62, outerring 60, as well as comb-shaped members 8, the shape of this conductivepattern 31 being essentially a projection of the free-standingoscillating structure 6 on the surface of the substrate 2. Connectingthis conductive pattern 31 to the same potential as the free-standingoscillating structure 6 suppresses forces perpendicular to the substrate2 between the ring resonator 4 and the surface of the substrate 2leading to a resonant frequency which is independent of the directvoltage component.

FIGS. 10 a and 10 b show further improvements of the micromechanicalring resonator 4 according to the present invention which allow thetemperature coefficient of the resonant frequency to be reduced to avalue close to zero. Two main factors determine the temperaturecharacteristics of the ring resonator. Firstly, Young's modulus E of thematerial used to realize the vibrating structure decreases withincreasing temperature resulting in a reduced stiffness of the springelements 62 and therefore a lower resonant frequency. Secondly, due tothermal expansion, the diameter of the ring will increase withincreasing temperature resulting in an increased mass moment of inertiaof the structure, which, in turn, also reduces the resonant frequency.

Different thermal expansion coefficients of different materials can beused to introduce a compensation mechanism 65, as sketched in FIGS. 10 aor 10 b. As shown in FIGS. 10 a and 10 b, a plurality of thermallycompensating members 65 (only one is shown in the Figures) are attachedto the outer ring 60. These thermally compensating members 65 aredesigned to alter the mass moment of inertia of the free-standingoscillating structure 6 as a function of temperature so as tosubstantially compensate for the effect of temperature on the resonantfrequency of the resonator 4. To this effect, the members 65 include aweight member 66 connected to the outer ring 60 by means of a connectingmember 67 comprising first and second layers 68, 69 made respectively offirst and second materials having different thermal coefficients. Thematerials are chosen so that the thermal expansion coefficient α_(th1)of the first layer 68 is smaller than the thermal expansion coefficientα_(th2) of the second layer 69. In a preferred embodiment, the firstmaterial is silicon and the second material is a metal, preferablyaluminium.

The design of the mechanism 65 according to FIG. 10 a is such that, withincreasing temperature, the connecting member 67 straightens due to thedifferent thermal expansion of the first and second layers 68, 69. As aconsequence, the weight members 66 move towards the centre of the ring,i.e. closer to the axis of rotation O of the oscillating structure 6,thereby reducing the mass moment of inertia of the ring resonator,resulting in an increase of the resonant frequency which substantiallycounteracts the effect of the Young's modulus and the thermal expansionof the ring on the resonant frequency. Such thermal compensationmechanisms can alternatively be attached to the outer side of the ring60, as shown in FIG. 10 b, or to some other part of the free-standingoscillating structure 6 so as to alter its mass moment of inertia as afunction of temperature. The layout and fabrication of the members 65have to be realized so that the weight members 66 move towards the axisof rotation O of the ring resonator when temperature increases.

Having described the invention with regard to certain specificembodiments, it is to be understood that these embodiments are not meantas limitations of the invention. Indeed, various modifications and/oradaptations may become apparent to those skilled in the art withoutdeparting from the scope of the annexed claims.

1. A time base comprising a resonator and an integrated electroniccircuit for driving said resonator into oscillation and for producing,in response to said oscillation, a signal having a determined frequency,said resonator being an integrated micromechanical ring resonatorsupported above a substrate and adapted to oscillate around an axis ofrotation substantially perpendicular to said substrate, said ringresonator comprising: a central post extending from said substrate alongsaid axis of rotation; a free-standing oscillating structure connectedto said central post and including an outer ring coaxial with said axisof rotation and connected to said central post by means of a pluralityof spring elements; and electrode structures disposed around said outerring and connected to said integrated electronic circuit, wherein saidfree-standing oscillating structure further comprises a plurality ofthermally compensating members, said thermally compensating membersbeing distinct from the spring elements and adapted to alter a massmoment of inertia of said free-standing oscillating structure as afunction of temperature so as to compensate for the effect oftemperature on the resonant frequency of the ring resonator.
 2. The timebase according to claim 1, wherein said thermally compensating membersare attached to said outer ring on its inner side or outer side.
 3. Thetime base according to claim 1, wherein said free-standing oscillatingstructure comprises at least a first pair of diametrically opposedthermally compensating members attached to said outer ring.
 4. The timebase according to claim 1, wherein each of said thermally compensatingmembers comprises a weight member connected to said free-standingoscillating structure by means of a connecting member comprising firstand second layers made respectively of first and second materials havingdifferent thermal coefficients, said connecting member being adapted togradually bring said weight member closer to said axis of rotation whentemperature increases, thereby reducing the mass moment of inertia ofsaid free-standing oscillating structure.
 5. The time base according toclaim 4, wherein said first material is silicon and said second materialis a metal.
 6. The time base according to claim 5, wherein said metal isaluminium.
 7. The time base according to claim 1, wherein said substrateand said ring resonator are made of silicon material.
 8. The time baseaccording to claim 1, wherein the thermally compensating members areattached to the free-standing oscillating structure at one end, and arefree at their other end.
 9. A resonator in the form of an integratedmicromechanical ring resonator supported above a substrate and adaptedto oscillate around an axis of rotation substantially perpendicular tosaid substrate, said ring resonator comprising: a central post extendingfrom said substrate along said axis of rotation; and a free-standingoscillating structure connected to said central post and including anouter ring coaxial with said axis of rotation and connected to saidcentral post by means of a plurality of spring elements, wherein saidfree-standing oscillating structure further comprises a plurality ofthermally compensating members, said thermally compensating membersbeing adapted to reduce a mass moment of inertia of said free-standingoscillating structure with increasing temperature so as to compensatefor the effect of temperature on the resonant frequency of the ringresonator.
 10. The resonator according to claim 9, wherein saidthermally compensating members are attached to said outer ring on itsinner side or outer side.
 11. The resonator according to claim 10,wherein at least two diametrically opposed thermally compensatingmembers are attached to the outer ring.
 12. The resonator according toclaim 9, wherein each of said thermally compensating members comprises aweight member connected to said free-standing oscillating structure bymeans of a connecting member comprising first and second layers maderespectively of first and second materials having different thermalcoefficients, said connecting member being adapted to gradually bringsaid weight member closer to said axis of rotation when temperatureincreases, thereby reducing the mass moment of inertia of saidfree-standing oscillating structure.
 13. The resonator according toclaim 12, wherein said first material is silicon and said secondmaterial is a metal.
 14. The resonator according to claim 13, whereinsaid metal is aluminium.
 15. The resonator according to claim 9, whereinsaid substrate 2 and said ring resonator 4 are made of silicon material.16. A time base comprising a resonator and an integrated electroniccircuit for driving said resonator into oscillation and for producing,in response to said oscillation, a signal having a determined frequency,said resonator being an integrated micromechanical ring resonatorsupported above a substrate and adapted to oscillate around an axis ofrotation substantially perpendicular to said substrate, said ringresonator comprising: a central post extending from said substrate alongsaid axis of rotation; a free-standing oscillating structure connectedto said central post and including an outer ring coaxial with said axisof rotation and connected to said central post by means of a pluralityof spring elements; and electrode structures disposed around said outerring and connected to said integrated electronic circuit, wherein saidfree-standing oscillating structure further comprises a plurality ofthermally compensating members, said thermally compensating membersbeing adapted to reduce a mass moment of inertia of said free-standingoscillating structure with increasing temperature so as to compensatefor the effect of temperature on the resonant frequency of the ringresonator.
 17. The time base according to claim 16, wherein thethermally compensating members are attached to the free-standingoscillating structure at one end, and are free at their other end.